and the"real" reason for it

Ride one, and ride a competitors and decide which you like. If you like the feel of the triple triangle, you proably can't go wrong buying one.

All true. It effectively shortens the rear triangle's tube lengths, and shorter tubes will flex less, other factors being equal. Whether it's a difference you can feel kinda depends on what you're riding for comparison. It definitely does NOT make the ride more plush, I think all can stipulate to that.
Allentown PA framebuilder John Stinsmen was building his custom Veloce road and especially track bikes with the same design long before GT glommed onto the idea. As were others, you can see "triple triangle" frames in "The Data Book '83 from the turn of the century -- the 19th century.
A GT product manager once admitted to me the biggest reason for the Triple Triangle design in MTBs is that they long built their BMX bikes that way, and they wanted to attract the grown-up BMXers who were familiar with the product. For that market, as important as how/where the seatstays cross, is the "GT" logo on the back end of the cut and capped extended top tube that hangs off the back of the seat cluster. The design also makes the frames recognizable at a distance, as in a race.